Encapsulated assemblages of thermoplastic films are known. For example, U.S. Pat. No. 4,262,161 discloses a solar cell assemblage prepared using a transparent film comprised of a block copolymer having at least two monoalkenyl arene polymer end blocks A and at least one polymer mid block B selected from the group consisting of substantially completely hydrogenated conjugated diene polymer blocks, ethylene-propylene polymer blocks and ethylene-butene polymer blocks between the cover plate and the electrical contact of the solar cell.
Even though this film provides good weather resistance for the resultant solar cell assemblage, it has been long desired to develop a new and novel film for use in all types of encapsulated assemblages which (1) is transparent to sunlight; (2) is resistant to ultraviolet degradation, (3) has good refractive index; (4) has low water absorption/permeation properties; (5) acts as an electrical insulator; (6) is heat sealable; (7) has a wide service temperature range, preferably from about -40.degree. C. to about 90.degree. C.; (8) has good adhesion, giving a peel strength of up to 30 pounds per inch with cohesive failure; and (9) can be extruded or curtain coated onto suitably prepared glass or silicone-based substrates.
Known encapsulant films have been highly susceptible to delamination from the base substrates and as a result have had low peel strength, in the range of less than 10 pounds per inch with adhesive failure. A need has long existed to prepare encapsulated assemblage with a film capable of achieving a peel strength of over 10 pounds per inch with cohesive, not adhesive, failure.
While polyvinylbutyral (PVB) and polyethylene/vinylacetate (EVA) are typical encapsulants, they have certain failures which have not been overcome prior to the development of the present encapsulant system. PVB is hygroscopic which requires careful control of temperature and humidity during storage and conditioning and during sheeting operations in a manufacturing environment. Failure to control the atmosphere results in poor laminations due to voids formed from outgassing of absorbed water. The moisture sensitivity of PVB is unacceptable in several product specifications. EVA has three basic problems, first a need for peroxide catalyzed crosslinking to achieve creep resistance. Since the peroxide in the film is relatively volatile, its concentration in the film in free sheet form, which determines the ultimate level of crosslinking, can vary, (see description in I. E. du Pont de Nemours & Co., Technical Guide, Elvax 150 EVA-Solar Photovoltaic Module Pottant, J. D. Pomije). Thus, the degree of crosslinking, can vary in EVA and it is difficult to test this parameter in certain in-line production processes. A second concern is the time needed to achieve sufficient crosslinking of EVA, which is not consistent with a high throughput production lamination cycle time. Thirdly, while peroxide is volatile in a free-standing film, it is not so volatile that all unused peroxide is removed during lamination which could compromise the longevity of some module packaging materials like edge sealants.
The problems alluded to with PVB and EVA have led to the search for an alternative encapsulant that is easily controlled in a high throughput manufacturing setting and is at least at reliable as PVB and EVA.
U.S. Pat. No. 3,984,369 discloses a composition which could be used as a sealant and which had good substrate/sealant adhesion, good UV stability, good peel strength in tests against glass, and useful hardness, tensile and flexibility properties. This novel sealant comprised:
(a) about 3 to 30 percent by weight of an elastomeric ABA type poly(.alpha.-monoalkenyl arene)/hydrogenated poly(conjugated diene) block copolymer having at least two poly(.alpha.-monoalkenyl arene) A blocks wherein the average molecular weight of the arene blocks is 4,000 to 50,000; the average molecular weight of the poly(conjugated diene) B block is 18,000 to 250,000; the conjugated diene block contains at least 20%, 1,2 bonds prior to hydrogenation; and at least 98% of the double bonds present in the conjugated diene blocks are saturated during hydrogenation,
(b) about 2 to 40 percent by weight of a butyl rubber;
(c) about 1 to about 70 percent by weight of an oil having a solubility parameter of from about 6 to about 8;
(d) about 1 to about 50 percent by weight of an adhesion promoting resin having a solubility parameter of from about 8 to about 12;
(e) about 0 to about 70 percent by weight of an inorganic filler; and
(f) about 0.01 to about 2.0 percent by weight of an oxidation stabilizer.
Even though this composition had good UV stability and it does not undergo covalent crosslinking, the adhesion of the compositions containing little or no butyl rubber was poor, giving only low peel strength and failing by the undesirable adhesive failure mechanism. The reason for this is that the ABA type polymers are very strong, making it impossible to achieve a composition which will fail by the desired cohesive failure mechanism. Compositions based on an ABA type polymer can be quite high in viscosity making them difficult to extrusion coat onto a substrate. An alternative method of applying these high viscosity compositions is to mix the composition in an extruder, grind it to form a powder, which is then applied directly to the desired object and heat fused onto the object assemblage or system forming the desired encapsulant coating. One such process is described in U.S. Pat. No. 4,207,359. This method has involved a variety of steps and it has been long desired to have a composition which can be extruded directly onto the desired substrate.
Another novel composition, taught in U.S. Pat. No. 4,296,008, involved a sealant composition with improved adhesion and melt flow properties based on the lower viscosity A'B'/ABA type block polymer. This sealant composition comprised:
(a) 100 parts by weight of a selectively hydrogenated block copolymer component comprising and A'B' block copolymer and an ABA type multiblock copolymer having at least two end blocks A and at least one mid block B wherein the A' and A blocks are monoalkenyl arene polymer blocks and the B' and B blocks are substantially completely hydrogenated conjugated diene polymer blocks, the number average molecular weight of the A' and A blocks are between about 3,000 and about 7,000 and the monoalkenyl arene content of the multiblock copolymer is between about 7% and about 22% by weight, wherein the weight ratio between the A'B' block copolymer and the ABA type multiblock copolymer is from about 20:80 to about 60:40;
(b) about 50 to about 350 parts by weight of a tackifying resin compatible with block B;
(c) about 0 to about 100 parts by weight of a plasticizer; and
(d) about 0.1 to about 10 parts by weight of a silane coupling agent.
To insure good adhesion between a pottant based on an ABA type block copolymer and a substrate, the pottant must contain an adhesion promoting tackifying resin. To maintain good adhesion and impact resistance in the solar cell at low temperatures, the pottant must have a low glass transition temperature (T.sub.g). The T.sub.g of the hydrogenated polybutadiene B block of the block copolymer is -58.degree. C. by differential scanning calorimety (DSC). The T.sub.g of a typical 95.degree. C. softening point tackifying resin is about 45.degree. C. The T.sub.g of the pottant comprised of a blend of ABA type block copolymer and tackifying resin will be intermediate between the T.sub.g 's of the polymer and the resin and will depend on the relative proportions of polymer and resin in the blend. If it is necessary to maintain low T.sub.g in the pottant (e.g. -30.degree. C.), only a limited amount of the high T.sub.g resin can be included in the composition. The relatively high percentage of ABA type block copolymer required in the composition to insure low T.sub.g causes the melt viscosity of the pottant to be very high, making it difficult to extrusion coat the pottant onto a surface.
To overcome this problem of high viscosity, it was discovered that the block copolymer used in the pottant must be of the ABA/A'B' type. The A'B' type diblock copolymers have much lower melt viscosity than do the ABA type multiblock copolymers. It was found that by using a block copolymer containing about 30/70 ratio of ABA/A'B' block copolymers, a pottant could be made containing a limited amount of tackifying resin which had good adhesion and low T.sub.g and which could be readily extrusion coated onto a substrate. Even though the novel composition could be extruded and applied to a variety of substrates such as glass or silicone release coated paper, the discovery of this novel composition was not enough to enhance the development of encapsulant systems or encapsulant assemblages, in general.
To overcome these described disadvantages, a silane treatment was developed to be used in combination with this novel extrudable composition. The development of a silanization procedure, i.e. a pretreatment procedure that promotes the adhesion of this novel composition to a substrate so the encapsulant can endure prolonged humidity and freezing cycles, was a critical discovery. This discovery resulted in a method for preparing encapsulant systems and encapsulated assemblages with improved durability, as well as a variety of other features. Furthermore, this novel method of improving the adhesion of this novel thermoplastic film to many substrates can be extended to a variety of end uses. For example, automotive safety glass can be prepared with the novel silane pretreatment and the novel film to enhance passenger safety and add an additional cover to the glass to prevent harm to passengers and drivers in the car, should they contact the automotive glass. Similarly, this novel film can be applied to glass containers to prevent shattering, such as for pressurized soft drink bottles as well as for non-pressurized bottles such as milk bottles.